This invention generally concerns a method and system for testing the accuracy of a thermocouple probe used to measure the temperature of a molten metal, such as steel, in a continuing casting operation.
Probes (such as Accumetrix.RTM. probes manufactured by Vesuvius Crucible Company) for monitoring temperature conditions in metallurgical processes such as steel casting are widely used in the prior art. These probes typically comprise a thermocouple formed from a junction of platinum and rhodium wires encased in a protective ceramic jacket. An example of such a device is the Accumetrix.RTM. probe manufactured and sold by the Vesuvius Crucible Company located in Pittsburgh, Pa. In operation, such probes may be inserted into molten steel contained within the tundish of a continuous casting machine. The thermocouple generates a millivolt potential that is converted into a temperature reading by a voltmeter. Because the physical properties of the resulting steel products are largely dependent upon the degree of superheat of the molten steel before solidification, it is critical that the temperature of the steel in the tundish by measured both reliably and accurately.
Systems for determining the reliability of the millivolt output of a thermocouple in a steel-fabricating environment are known in the prior art. U.S. Pat. No. 5,549,820 assigned to the Vesuvius Crucible Company describes and claims a system for determining whether or not the electrical output of a thermocouple probe is spurious due to, for example, a bad connection somewhere in the signal transmission circuit. This particular system works by continuously monitoring the impedance of the thermocouple loop circuit during the operation of the thermocouple, thereby allowing it to detect the advent of a spurious signal immediately upon its occurrence during the steel casting operation. However, as useful as such a system is in determining the electrical integrity of the thermocouple loop circuit, it cannot determine the point in time when the accuracy of the temperature reading first begins to drift or deteriorate near the end of the life of the thermocouple. But before this particular shortcoming of the prior art can be fully appreciated, both the structure and operation of the thermocouple probes used in steel making plants must be understood in greater detail.
The thermocouple probes used to measure the temperature of molten steel in a tundish generally comprise a "hot" junction of rhodium and platinum type B wires that generate a voltage when heated due to the dissimilarity of the metals at the junction. The platinum and rhodium wires leading away from the hot junction are disposed within separate bores in an electrically insulative rod that is covered by an inner alumina sheath. The alumina sheath is in turn covered by an outer molybdenum sheath due to molybdenum's high melting point and excellent thermal shock resistance. In use, the hot junction end of the molybdenum covered probe is inserted within a refractory protector tube immersed in the molten steel. After the hot junction end of the probe has obtained thermal equilibrium with the surrounding molten steel through the walls of the protector tube, the resulting millivolt output is measured and converted into a temperature.
Over time, the temperature readings of the probe will begin to drift away from an accurate measurement. Such drift may occur slowly as a result of either a natural aging process, wherein platinum and rhodium atoms diffuse into one another at the hot junction, or more quickly as a result of the inadvertent bending of the probe by the system operator after removal from the protector tube. Such bending is made possible by the fact that molybdenum becomes ductile at temperatures above 1850.degree. F., thus rendering the thermocouple assembly quite flexible. If the bending is severe enough to break the inner, double-bore alumina insulator, the inventors have observed that the combination of localized stresses and "line of sight" exposure of the bent thermocouple wires to the molybdenum sheath forms a contamination zone on the wires that act as a secondary thermocouple whose output subtracts from the millivolt output of the hot junction at the head of the thermocouple, thereby degrading the accuracy of the temperature reading inferred from the net millivolt output. The applicants have also observed that the secondary subtractive voltage generated by the contaminated zones increases fairly rapidly with time and also with temperature, ultimately rendering the probe completely useless.
Of course, a higher degree of accuracy could be obtained by replacing the thermocouple probe after a one-time use in a particular tundish of steel. But, such a solution would be expensive, as the platinum/rhodium junction and molybdenum sheath used in such probes is costly. Alternatively, the probe could be tested between tundishes by reheating in a device with a known flame temperature. But such testing would be impractical in a typical steel casting operation and would add significantly to the cost of temperature measurement. Additionally, such tests would not tell the operator the exact time that the probe became inaccurate. For this reason, the present solution favored by the prior art is the use of a second "reference" thermocouple probe to verify the results obtained by an in-use thermocouple probe. But once again, such a solution is expensive as it requires the use of two relatively costly temperature sensing components, both of which have a limited lifetime due to the harsh environment imposed by molten steel.
Clearly, there is a need for a method and system for verifying the reliability of the temperature readings of a thermocouple probe which allows the probe to be reused, and continually monitors the accuracy of the probe during use, but does not require the use of a verifying, second probe. Ideally, such a system and method would be compatible for use with existing thermocouple probes without the development or installation of expensive new components. Finally, such a method and system would be sensitive enough to detect the occurrence of inaccurate temperature readings as soon as they begin even when the probe is in use in a pool of molten steel.